This project represents a continuing investigation of the biochemical nature of Factor V found in the plasma of man and cow. This labile protein plays an important role in the key reaction of conversion of prothrombin to thrombin. Its exact role has not been clearly delineated due to problems in obtaining pure, stable samples for study. Current evidence obtained in this laboratory have shown that human as well as bovine plasma Factor V is a high molecular weight protein, with values close to 1 times 10 to the 6th power daltons if high concentrations of the protease inhibitor, DFP, is present in the initial collection media, particularly of the bovine samples. It is the object of this investigation to define in biochemical terms the characteristics of this very high molecular weight Factor V, specifically its amino acid composition, carbohydrate content, N-terminal amino acids, and occurrence of subunits. Subsequent to establishment of these characteristics, by conversion of this high molecular weight V to an "activated" form by action of thrombin will be studied. It is anticipated that this factor V will proceed through a series of intermediates and that small molecular weight glycopeptide will be released in the activation process. The latter possibility will be investigated in detail.